1. Field of the Invention
The present invention relates to a gas detection apparatus which includes a gas sensor element and which detects the presence/absence or concentration of a specific gas in a gas to be measured (hereinafter referred to as “measured gas”), and to a gas-sensor control circuit for controlling the gas detection apparatus. Moreover, the present invention relates to an inspection method for inspecting the gas detection apparatus for the presence of short circuits.
2. Description of the Related Art
In a known combustion control scheme for an internal combustion engine such as a gasoline engine, in accordance with the concentration of a specific gas contained in exhaust gas, the quantity of fuel supplied to the engine is subject to feedback control so as to control the air-fuel ratio of a gas mixture of air and fuel supplied to the engine. The object of such control is to reduce CO, NOx, and HC emissions in the exhaust gas.
A known gas sensor element used for such air-fuel ratio control is a full range air-fuel-ratio sensor (hereinafter also referred to as a “UEGO sensor”) in which two sensor cells (a pump cell and a detection cell) each including a solid electrolyte member formed of, for example, zirconia, and two electrodes disposed on opposite surfaces thereof are disposed to face each other. This UEGO sensor includes these two sensor cells, and a measurement chamber which communicates with a measured space (a space where measurement is performed) via a diffusion resistance. The UEGO sensor can detect the oxygen concentration of a measured gas introduced into the measurement chamber via the diffusion resistance.
Incidentally, when a portion (e.g., a wire or a connector) which is electrically connected to an electrode of a detection cell is, for any reason, short-circuited to the power-source potential or the ground potential, current flows through the detection cell in an improper direction, and so-called blackening, which is a phenomenon in which oxygen ions disappear from the solid electrolyte constituting the detection cell, occurs, so that the measurement characteristics of the gas sensor element deteriorate.
Further, a gas sensor element is known in which one of electrodes which constitute a detection cell is disposed outside a measurement chamber in a closed space isolated from the outside, and a small, constant current is supplied to the detection cell so as to cause the closed space to function as a reference oxygen chamber. In such a gas sensor, when the electrode disposed in the closed space or a portion which is electrically connected to the electrode is short-circuited to ground, excessive current flows through the detection cell, and oxygen is excessively pumped. As a result, the pressure of the reference oxygen chamber increases, thereby possibly rupturing the gas sensor element.
In order to cope with such a problem, a gas detection apparatus has been proposed which uses a gas sensor element of the above-described type and which performs diagnosis as to the presence or absence of short-circuiting of the wire, etc., to ground or the power-source.
In the gas detection apparatus disclosed in Japanese Patent Application Laid-Open (kokai) No. 2003-90821, the potentials of portions which are electrically connected to the electrodes of the sensor cells of the gas sensor apparatus are measured for such short-circuit inspection. A determination as to, for example, whether the measured potentials fall within a predetermined range is made so as to determine whether any of the portions electrically connected to the electrodes of the sensor cells is short-circuited to the power-source or to ground.
However, the gas detection apparatus disclosed in the publication detects anomalous conditions, such as a short-circuit, in a state in which the gas detection apparatus is performing a predetermined operation after activation of the gas sensor element. Therefore, the gas detection apparatus cannot cope with a problem in which current flowing in an improper direction adversely affects the gas sensor element at the time of startup of the gas detection apparatus. Specifically, in a case where, for some reason, a portion electrically connected to one electrode of the detection cell is short-circuited to ground before startup, when the gas detection apparatus, including a circuit which is connected to the detection cell for short-circuit inspection, is started, current immediately starts to flow in an improper (reverse) direction through the detection cell, from an output terminal of the circuit connected to the other electrode of the detection cell to the one electrode short-circuited to ground. Therefore, even if the gas detection apparatus detects that the portion electrically connected to the one electrode is short-circuited to ground and immediately cuts off power to the gas detection apparatus, the gas detection apparatus cannot prevent an adverse effect on the gas sensor element exerted due to current which has already flowed through the detection cell in an improper direction before the power is cut off.